Pressurized sealing means for a hydraulic turbodrill

ABSTRACT

A hydraulic motor of the turbodrill type including a housing, a rotor shaft rotatably mounted via bearings in said housing and connectible to a drill bit to transmit driving/torque thereto, sealing means between the rotor shaft and the housing arranged to oppose access of drilling mud to said bearing means, said sealing means comprising pressurized inner and outer sealing devices.

United States atet 1 Mayall PRESSURIZED SEALING MEANS FOR A HYDRAULIC TURBODRILL William Mayall, 6 Arundel Garden, Ash Ln., Rustington, Sussex, England Filed: June 29, 1971 Appl. No.: 158,039

Inventor:

[30] Foreign Application Priority Data June 30, 1970 Great Britain 31,751/70 Apr. 30, 1971 Great Britain 12,446/71 US. Cl. 415/113, 415/502 Int. Cl F0ld 11/00 Field of Search 415/113, 107, 502;

References Cited UNITED STATES PATENTS 2,806,672 9/1957 Selberg et a1 415/113 [451 July 31, 1973 3,594,106 7/1971 Giarrison 415/202 3,630,634 12/1971 Mayall 415/170 Primary Examiner-C. J. Husar AttorneyBacon & Thomas [57] ABSTRACT 5 Claims, 13 Drawing Figures 250 5556 50 j 4 /4 2 WWW/4 km d 1 ll/L/lsfl AV/ ///V /l I ll . PATENTEI] JUL 3 1 I975 SHEET 3 OF 7 uvvgrvroxe 5y WILL/HM MAVALL A 770/?NEV5 PATENTED JULB 1 I975 3 749 51 1 SHEET u UF 7 N VEN TOR W/LL/HM MA mLL A ORWEVS PATENTEDJUL31 m5 3.749.511

SHEET 5 OF 7 Q INVENTOR WILL/AM MA WILL Q/ f7 TTOPNEYS PATENTEB JUL31 1975 3 749,551 1 SHEET 6 OF 7 fig 0 9/ 92 3/,

INVENTOI? W/Lu/m Mn WILL BY A TTORNE Y5 PRESSURIZED SEALING MEANS FOR A HYDRAULIC TURBODRILL This invention concerns rock drills and more particularly drills for penetrating the earth's crust to great depths, for instance to reach oil or gas-bearing strata. Commonly, such drilling is accomplished by the use of a drilling rig adapted to rotate a suitable drill bit by rotation of a so-called drill-string of coupled pipes that extend from the rig on the surface down to the drill bit which is fixed at the bottom of the lower-most pipe of the drill-string. Chips and rock particles produced by the action of the rotating bit on the borehole bottom are flushed to the surface via the annular space between the drill-string and the borehole wall by a stream of liquid that is conveyed to the bottom of the borehole through the connected pipes constituting the drillstring. This liquid, usually called mud, also serves the purpose of cooling the drill bit.

The torque needed to rotate the drill bit is substantial and, as indicated, is commonly transmitted to the drill bit through the drill-string from the rig on the surface; the energy stored in the drill-string when transmitting this torque is therefore considerable, especially when drilling at great depths, giving rise to difficulties when the bit penetrates discontinuities that cause fluctuations in the torque requirements of the bit. Moreover, the transmission of the required torque from the rig to the bit requires the drill-string pipes to be of substantial wall thickness and rigidity and thus both heavy and expensive. The requirement for the drill-string pipes to have substantial wall thickness conflicts with the other requirement of these pipes, namely to have a large bore cross section for conducting the mud in large volumes to the bottom of the borehole to accomplish its bitcooling and scavenging functions. In addition, ifa rectilinear borehole is to be cut, the drill-string pipes must be straight and accurately formed so as to transmit torque without deflection that would cause the drill bit to wander and produce a non-rectilinear borehole; on the other hand the formation of a borehole having a planned deviation from the rectilinear, as is sometimes desirable, is difficult if not impossible.

lt has therefore been recognised that many of the disadvantages and design compromises in drills of the above described construction might be avoided by generating the required torque for rotating the drill bit by means disposed adjacent the drill bit and powered by energy conducted to such means. The so called turbodrill" is one well-known proposal for accomplishing this objective, the turbodrill comprising a hydraulic motor in the form of a turbine adapted to be fitted to the bottom of a drill-string and to extract energy from the mud flowing down the latter so as to produce torque to rotate the drill bit relative to the drill-string. The art is replete with proposals for turbodrill constructions but, so far as I am aware, no turbodrill yet proposed has demonstrated an ability to compete in practical circumstances with a drill of the conventional construction discussed above.

Drilling mud is commonly a highly abrasive slurry or suspension of sand and rock particles: the mud is continuously circulated through the drill-string, the borehole and a settling pond or the like in which the bulk of the larger chippings are deposited. This extremely abrasive fluid is understandably damaging to any relatively moving surfaces between which it may penetrate and, as previously mentioned, the mud at the bottom of a deep borehole is under a very substantial hydrostatic pressure so that its power of penetration into bearing structures and between relatively moving parts is very high. The bearings of a hydraulic motor for a drill are therefore prone to become contaminated with the highly abrasive mud that is used to drive the motor; since these bearings are subject to extremely high loadings and large load variations, it will be understood that bearing failure has been a notable cause of unreliability in hydraulic motors of the turbodrill type so far proposed.

The present invention provides a hydraulic motor for driving a rock drill, comprising a housing connectible to a drill-string and a rotor shaft rotatably mounted via bearing means in said housing and connectible to a drill bit, sealing means between the rotor shaft and the housing on each side of said bearing means, the space containing the bearing means between said sealing means being adapted to be filled with lubricant, at least one cylinder housing a piston being in communication with said space, and means being provided for leading drilling mud, at a pressure at least as high as the mud which will be present outside of the said sealing means in operation, to the side of said piston remote from said space so as to pressurise the lubricant therein. The fact that the drilling mud led to the said cylinder is at a pressure at least as high as the mud which is on the outside of the said sealing means in operation, so that the lubricant in its turn is maintained at a pressure at least as high as the mud outside the sealing means, keeps the mud out of the sealing means and out of the space containing the bearing means. In a preferred arrangement the said cylinder is in communication with the said space containing the bearing means via an inlet to said space closely adjacent one of the said sealing means, preferably the lower one. Preferably means are also provided for leading pressurised mud to a point between the ends of at least one of the said sealing means (preferably that adjacent which the said lubricant inlet is situated) and a path for the leakage of mud out of the said housing is provided from that point to an outlet outside of the sealing means so that mud, rather than lubricant, will be caused to leak under the pressure in the space containing the bearing means.

In one form of the invention the cylinder and piston assembly for pressuring the lubricant is of annular configuration, the cylinder being an annular one defined between the said rotor shaft and its housing. Preferably however the cylinder and piston assembly is of conventional configuration and is positioned near to what will be the top end of the motor in use, above the rotor, that chamber of the cylinder which is intended to contain lubricant in operation being connected to the said space containing the bearing means via a conduit extending along and within the rotor. The cylinder is preferably dimensioned to be capable of housing a substantial quantity of lubricant, eg about 9 gallons of grease, so that despite the small leakages which must inevitably occur in operation it will only need re-charging at infrequent intervals.

It will be appreciated that thrust bearing means must be provided between the housing and the rotor assembly of a hydraulic motor according to the invention, capable of supporting thrust loads applied in either direction and which are such as to fluctuate and repeatedly reverse during many practical drilling circumstances.

Preferably a hydraulic motor according to the invention incorporates at least one thrust bearing structure of the kind described and claimed in U.S. Pat. No.

be described by way of example and with reference to the accompanying drawings, in which:

FlGS. la to 1d are a view in axial cross-section of a hydraulic motor according to the invention; and

FIGS. 2 to are transverse cross-sectional views on the respective lines indicated in FIG. 1. i

The general arrangement of the hydraulic motor will first be briefly described with reference to FIGS. 10 to 1d.

The motor comprises a housing in the form of a cylindrical casing tube 1 which extends from end to end of the motor and is provided with an internally screwthreaded fitting 2 at its top end for attachment to the bottom pipe of a drill-string. The lower end of the casing tube is internallyscrew-threaded at 3 for attachment to an endfitting 4. The casing tube 1 will not, of course, rotate in operation and it constitutes the stator of the motor assembly.

A rotor assembly is supported for rotation within the casing tube 1 and includes a rotor shaft 5 which is mounted in a sealed bearing system over the great part of its length. The rotor shaft 5 is tubular and has its upper end connected by six drive pins 6 to the bottom end of the drive means assembly of the rotor, whilst the lower end of the rotor shaft is radially enlarged at 7 and is then formed as an integral end fitting 8 formed with an A.P.l. internal thread 9 for the connection of a drill bit (not shown).

The construction of the hydraulic motor will now be described in detail beginning at its top end,.i.e. the lefthand end of FIG. la.

. automatically operative to provide an exit for the column of mud in the drill-string when the mud becomes de-pressurized, e.g. when the drill-string is to be withdrawn from the borehole. The dump valve comprises a body 11 secured in the fitting 2 by a lock nut 12 and sealed to the interior of the fitting 2 by sealing rings 13, the body 11 including a transverse portion 14 formed with three exit passages 15 for mud, leading via an annular recess 16 in the inside wall surface of the fitting 2 to an exit opening 17 extending through such wall. The transverse portion 14 of the dump valve casing is also provided with three passages 18 for the flow of pressurized mud therethrough, which are always open. Entry of mud to the exit passages 15 is however prevented, when the mud is pressurized, by a valve member 19 slidably mounted in the dump valve body and provided with a central plug 20 engageable in a central aperture 21 of the transverse portion 14 of the body, leading to the exit passages 15. Tungsten carbide wear rings 22 and 23 are provided on the valve member 19 and the transverse portion 14 of the dump valve body respectively, which inter-engage when the valve is closed, and a rubber sealing ring 24 which seals against the wear ring 23 is mounted around the plug member 20. The valve member 19 is urged upwardly in the dump valve body by a helical compression spring 25 so as to cause the valve to open automatically when the mud is depressurized, but is held in its closed position, against the action of the spring 25, by pressurized mud. Passages defined between fins 26 of the valve member permit the flow of pressurized mud past it and through the passages 18 in the transverse portion 14 of the dump valve body during operation.

The next part of the motor below the dump valve 10 is a cylinder and piston assembly whose purpose is to supply pressurized lubricant, more specifically grease, to the seals and bearings lower down the motor. The cylinder 27 of this assembly (only a small part of whose length is shown in FIG. 1a) is mounted at its lower end from a sleeve 28 to which the cylinder is screwthreadedly secured at 29. The sleeve 28 is formed with radially extending fins 30 which locate against the inside of the casing tube 1 to centralise both the cylinder 27 and other parts which are mounted within the sleeve and are described hereinafter. The cylinder 27 has welded to its top end a conical fitting 31 formed with an axial passage 32 for the flow of pressurized mud into the cylinder,and the cylinder contains a piston 33 arranged to be urged downwardly in the cylinder by the mud pressure. In use, the whole of the cylinder space 34 below the piston 33 will be filled with grease for pressurized application to the seals and bearings of the motor as previously mentioned.

The cylinder 27 is located at its top end by a locking sleeve 35 screwed in the casing tube and integrally connected to the fitting 31 by webs 36 defining mud flow passages therebetween. The sleeve 35 is connected to the casing tube by a left-hand thread in case the cylinder 27 should tend to rotate clockwise (as seen from above) during operation.

The sleeve 28 on which the cylinder 27 is mounted I houses and centralises the top end of the rotor assembly of the motor, in the form of a shaft 37 having a top end cap 38 screw-threadedly secured thereto at 39. The cap 38 is formed with an aperture 40 for the passage of pressurized grease from the cylinder space 34 into a bore 41 of the shaft 37. Pressurized grease can also pass into the narrow annular gap between the outside cylindrical surface of the cap 38 and the inside surface of the sleeve 28, to lubricate bearing means for the top end of the shaft 37, provided within the sleeve 28. Such bearing means takes the form of a hardened steel sleeve 42 mounted and secured in a recess within the sleeve 28 and rotatable on a bearing sleeve 43 secured on the shaft 37 by the end cap 38. A seal structure generally indicated at 44 (see also FIG. 2) is secured between a shoulder 45 of the sleeve 28 and a lock nut 46 screw-threadedly mounted in the lower end of the sleeve, and is formed by two axially spaced packs 47 of pairs of inner and outer split rings 48 and 49. Each pack 47 contains eight such pairs of split rings and the rings of each pair made a close fit one within the other (see FIG. 2). The splits 50 of the rings of each pair are slightly circumferentially spaced from one another and the two rings are fixed together at the interface 51 extending between the slits. The connection between the rings can be by brazing, welding or soldering depending on the material from which they are formed. The successive pairs of rings of each pack have their pairs of splits 50 indexed out of alignment. In the particular arrangement shown in FIG. 2 the splits 50b of the second pair of rings (from the top) are indexed 180 away from the splits 50 of the top pair, whilst the splits of the remaining pairs of rings are positioned as indicated by the successive reference numerals 500 to 50h in FIG. 2.

Each outer ring 49 is formed with a radial enlargement 52 (FIG. 2) engaging in one of six key-ways 53 formed in the sleeve 28 so that all of the rings 48 and 49 are thereby secured against rotation. The packs of rings 47 are separated by spacer rings 54 which float on a sleeve 55 secured on the shaft 37 and all of the spacer rings 54 and packs of sealing rings 47 are urged together axially by spring steel rings 56. The inner split rings 48 of each pack are urged into sealing engagement with the sleeve 55 on the shaft 37 by grease which is fed to the inside of the bearing sleeve 28. A distance piece 57 is provided between the uppermost spring steel ring 56 and the shoulder 45 of the sleeve 28.

The purpose of the seal structure 44 is to exclude pressurized mud from the bearing 42. The mud flows from the top of the cylinder 27 down the annular space defined between the cylinder and the casing tube 1 and thence through the passages defined between the fins 30 on the sleeve 28. When the mud reaches the lower end of the lock nut 46 on the sleeve 28, its flow divides into two parts, some of the mud flowing through an annular passage 58 into a second bore 59 in the shaft 37 and the remaining part of the mud flowing around the outside of the drive assembly of the rotor to drive the same in a manner to be described below.

Referring now to the drive assembly of the rotor in detail, the lower part of the shaft 37 constitutes an upper body portion of such assembly whilsta lower body portion thereof is constituted by a generally tubular member 60 screw-threadedly mounted at 61 on the lower end of the shaft 37. The structure of the drive assembly is also shown in the cross-sectional views of FIGS. 3 to 10, of which FIGS. 5 and 6 are taken in the same plane, FIG. 5 showing the rotor driving blades (to be described below) in their rest positions and FIG. 6 showing the blades in the positions which they occupy when the rotor is rotating. The shaft 37 and the tubular member 60, together with the various elements which they mount and support, form the top part of the rotor assembly of the hydraulic motor, the stator being constituted by the casing tube 1.

A ring of six rotor driving members in the form of blades 62 are mounted around the shaft 37 near to its lower end. Each blade 62 is provided with a cylindrical top mounting stud 63 which is rotatably received in one of six bores in a top blade mounting ring 64 rotatably mounted on the shaft 37 and sealed thereto by an O- ring 65. The ring 64 is secured in position on the shaft 37 by six Allen screws 66 mounted in a sleeve 67 splined on the shaft 37 at 68. The screws 66 thus hold the blade mounting ring 64 against rotation on the shaft 37; nevertheless, in order to permit the circumferential positioning of the blades so as correctly to locate their lower ends in further mounting means to be described below, the passages in the sleeve 67 through which the screws 66 pass are somewhat circumferentially enlarged. A nut 69 provided with an inset wear ring 70 is screwed down on top of the blade mounting ring 64 and held in position by a lock ring 71 which, like the nut 69,

is screwed on the shaft 37. A rubber sealing ring 72, formed with suitable arcuate cut-outs to receive the blade mounting studs 63, is inset in the bottom surface of the blade mounting ring 64.

It will thus be seen that the blades 62 are rotatable at their top ends in the mounting ring 64, by way of their mounting studs 63. At their lower ends, however, the blades are held against rotation in a manner now to be described.

A mounting and torsion shaft 73 is integral with the bottom end of each blade 62 and is received in one of a ring of six slots 74 in the outer surface of the lower tubular body part 60 of the drive assembly, the lower end extremities of the torsion shafts being received in bores 75 in the member 60 to locate the shafts circumferentially. The torsion shafts 73 are not intended to rotate when the motor is being driven but rather to twist so as to permit a limited degree of rotation of the blades 62. Each torsion shaft is therefore held against rotation by the engagement of a sector-shaped portion 76 thereof (see FIG. 9) in a correspondingly shaped aperture defined in its receiving slot 74 in the tubular member 60. As will be explained hereafter, the blades are urged to rotate clockwise (as seen from above) in operation and a suitably shaped wear piece 77 is provided on the appropriate side of the sector-shaped portion 76 of each torsion shaft 73, in the shaft-receiving slot of the member 60; the wear pieces 77 are easily replaceable when worn. It will thus be understood that each torsion shaft 73 is held against rotation at its lower end but is able to twist over the length of the shaft extending between the region of the FIG. 9 cross-section and the bottom end of the associated blade 62, so as to permit rotation of the blade about the axis of the torsion shaft as previously mentioned; the mounting studs 63 at the top ends of the blades are coaxial with the torsion shafts 73 and this axis is the twisting axis of the blade previously referred to herein.

The cross-sectional shape of the major part of the length of each blade 62 is that shown in FIGSv 3 and 4 and the twisting axes of the blades are indicated in those Figures at 78. The blades are shown in FIGS. 4 and 5 in their rest positions, i.e. when the rotor is stationary, but clockwise rotation of the blades during operation to drive the rotor brings them into the positions shown in FIGS. 3 and 6 in which their arcuate inner faces 79 precisely engage face-to-face with the surface of the shaft 37.

Referring now to the manner in which pressurized mud drives the rotor, it will be recalled that there is a flow of mud down the outside of the rotor assembly past the top end mountings of the blades 62, so that pressurized mud presents itself in the annular space 80 defined between the outer faces 81 (FIGS. 3 and 4) of the blades and the inside of the casing tube 1. The mud pressure causes clockwise rotation of the blades about their twisting axes as seen from above, accompanied by twisting of the torsion shafts 73 in the manner previously described, and a mud pressure in excess of a predetermined magnitude causes rotation of the blades to their positions indicated in FIGS. 3 and 6, with their inner faces 79 engaging the shaft 37. In these circumstances the torsion created in the shafts 73 is transmitted to the tubular body member 60 of the rotor assembly via the sector-shaped portions 76 of the shafts and the wear pieces 77, so as to drive the rotor assembly in clockwise rotation. The blades have rubber sealing strips 82 inset in their end faces 83, which strips are squeezed between their associated blades and the rear faces 84 of the adjacent blades when the blades are rotated inwardly by the mud pressure, so as to exclude mud from the spaces 85 (FIGS. 4 and which initially exist between the inside faces 79 of the blade and the shaft 37. A vent passage 86 is provided connecting the spaces 85 behind the blades to a region of reduced mud pressure below the drive assembly, to release the vacuum which will be created when the mud is depressurized, so as to enable the blades to open and thus to stop the rotor. Such a vent passage also prevents any buildup of high pressure mud behind the blades if the seals 82 should leak slightly, so as to maintain the necessary pressure differential across the blades during operation.

Means are provided for permitting, in flow-restricting fashion, the escape of pressurized mud from the annular space 80 surrounding the blades 62 when the rotor assembly is rotating. Thus each blade 62 is formed with an integral radially outwardly extending step 87 at its lower end, which step is formed with an axially extending slot 88 which receives a tungsten carbide insert 89 formed with a restricted passage 90 for the flow of mud thcrethrough. The slot 88 extends into the outer face 81 of the blade to some extent so that the insert 89 is somewhat inset relative to the face 81 (see FIGS. 3 and 4). As the blades rotate inwardly in operation each of the passages 90 gradually becomes fully aligned with one of a ring of six passages 91 defined in the tubular body member 60 and closed at their outer sides by strips 92 (FIGS. 1 and 8). Referring particularly to FIGS. 1 and 7 it will be seen that the inlets to the passages 91 of the body member 60 are defined by tungsten carbide inserts 93 and thence lead into oblique passages 94 for the flow of mud into the interior of the member 60, i.e. to the central space 95 thereof indicated in FIGS. 1 and 9. A rubber sealing ring 96 is inset in the top surface of the tubular body member 60 to seal against the bottom faces of the blades 62.

The mud which has been employed to rotate the rotor assembly loses only a relatively small part of its pressure in so doing, in this particular embodiment of the invention the pressure lost being in the region of 250 psi. This mud, having performed its driving function, flows freely through a central passage of the remaining length of the apparatus to the drill bit to perform the conventional cooling and scavenging functions. The rate of rotation of the rotor assembly, and thus of the drill bit, produced in the above described manner is of the order of 200 to 250 r.p.m., i.e. the desired and conventional rate of rotation of a drill bit in this kind of apparatus. At the same time, however, much of the power extracted from the mud is not lost but is converted into torque so that as well as rotating at the desired speed the drill bit is also provided with the necessary power.

The remaining length of the rotor assembly of the motor, below the drive assembly just described, is rotatably mounted in the casing tube 1 by sealed bearing means to be described below. This lower part of the rotor assembly is basically constituted by the tubular rotor shaft 5 previously referred to, which as already mentioned is connected by six drive pins 6 to the bottom end of the drive assembly of the rotor, specifically to the bottom end of the tubular member 60 constituting the lower body part of that assembly. The body member is further located relative to the top end of the rotor shaft 5 by a lock nut 97 screw-threadedly mounted on the rotor shaft 5 and carrying a ring of six Allen screws 98 engaged in threaded bores in the lower end of the member 60 to hold the latter firmly against rotation relative to the rotor shaft. The top end of the rotor shaft 5 is thus firmly secured against an internal shoulder 99 of the tubular member 60 and a rubber sealing ring 100 is provided at this point.

Because the drive assembly is of course rotatable within the casing tube 1, an annular passage 101 exists between the lower end region of the drive assembly and the inside of the casing tube, and there is pressurized mud in this passage in operation. This mud, which has not passed through the restricted passages 90 of the blades 62, is at the same pressure as when it enters the top of the motor, and this pressure is hereinafter referred to as top pressure. This mud thence fills a further annular passage 102 defined between an alongate nut 103 screw-threadedly mounted on the rotor shaft 5 and the inside of a connector member 104 whose function is to join together upper and lower parts of the outer casing tube 1. This top pressure mud eventually reaches the top end of a seal structure 105 whose purpose is to exclude mud from the radial bearings and thrust bearings of the rotor shaft. The seal structure 105 is of similar construction to the seal structure 44 previously described.

The purpose of the division of the casing tube 1 into two parts interconnected by the connector member 104 is to enable the upper part of the casing tube to be easily removed for access to the drive assembly, and the removal of that assembly from the rotor shaft 5 when desired.

The construction and manner of lubrication of the radial bearings and thrust bearings of the rotor shaft 5 will now be described with reference to FIG. 1.

It will be recalled that the shaft 37 constituting the upper part of the body of the drive assembly is provided with two internal bores 41 and 59 which respectively receive pressurized grease (at top pressure) from the cylinder 27 and top pressure mud. A tube 106 is secured in the lower end of the grease bore 41 of the shaft 37 and extends right through the interior of the rotor shaft 5 to the lower end thereof. Similarly a tube 107 is secured in the lower end of the mud bore 59 in the shaft 37 and again extends to the lower end of the rotor shaft 5. The purpose of the tube 106 is to deliver grease to the radial and thrust bearings to lubricate the same, whilst the purpose of the delivery of top pressure mud via the tube 107 to the lower end of the motor is to prevent leakage of lubricant from the bearings at such lower end, in a manner to be described below. Although not shown in the drawings, the tubes 106 and 107 do in fact cross each other in their passage through the rotor shaft.

The bearing assembly for the rotor shaft 5 includes a main axial thrust bearing structure generally indicated at 108, two radial sleeve bearings 109 and 110 below the thrust bearing structure, and a third radial sleeve bearing 111 above it. The seal structure 105 already mentioned, located above the radial bearing 111, is arranged to prevent the ingress of top pressure mud to the top end of the bearing assembly, whilst a second seal structure 112 (again similar to the seal structure 44) has the purpose of preventing the ingress of mud to the lower end of the bearing assembly.

The thrust bearing structure 108 is of the kind described in detail in U.S. Pat. No. 3,630,634. Thus such thrust bearing structure comprises inner and outer stacks of rings 113 and 114 secured respectively on the rotor shaft 5 and the casing tube 1 and having arcuately shaped annular corner faces defining six axially spaced tracks for sets of bearing balls 115, the arrangement being such that thrust is transmitted from the rotor shaft 5 to the casing tube 1 by shear stress in the bearing balls. The neighbouring pairs of track-defining rings 113 and 114 (both inner and outer) are separated by spacer rings 116 so dimensioned that the width of each ball track is slightly greater than the ball diameten'As a result, during the application of thrust to the bearing structure, the balls of each set are pinched between one pair of diametrically opposite track-defining rings whilst being free of the other pair. Each inner trackdefining ring 113 rotates within and in contact with its adjoining outer track-defining ring 114 so that the bearing structure is in fact ajournal bearing in addition to its main function as a thrust bearing.

Each of the radial bearings 109 to 111 comprises a hardened steel bearing sleeve secured to the casing tube 1 and rotatable on respective ones of a series of sleeves 117 mounted on the rotor shaft 5 and secured thereto by the elongate nut 103 previously mentioned. Referring more specifically to the upper radial bearing 111, the bearing sleeve of this bearing is secured in position by a locking sleeve 1 18 screw-threadedly secured to the casing tube 1 at 119, which locking sleeve also engages the top of the outer stack of rings 114 of the thrust bearing structure 108 to locate the same within the casing tube 1.

The two lower radial bearings 109 and 110 again comprise hardened steel bearing sleeves secured to the casing tube 1 within a pair of sleeve 120 and 121 which are themselves secured in position between the lower end of the outer stack of rings 1 14 of the thrust bearing structure and the end fitting 4 of the casing tube 1 and sealed to the latter by an O-ring 122. The lower seal structure 112 of the bearing assembly is mounted within the end fitting 4.

The grease for lubrication of the bearing assembly is delivered from the tube 106 through an end fitting 123 of such tube and thence through three passages 124, defined between the lower end of the rotor shaft 5 and the lowermost of the sleeves 117, to an annular channel 125 defined in a packing ring 126 at the top end of the seal structure 112. From this point the grease is able to flow in two directions. Firstly it flows around the outside of the seal structure 112 as far as the bottom end thereof, and secondly it passes up through the whole bearing assembly to the top end of the top seal structure 105. It will be appreciated that the grease delivered in this way is at top pressure because it is pressurized by top pressure mud applied to the piston 33 at the top of the motor. Thus the grease which reaches the top end of the top seal structure 105 of the bearing assembly is at the same pressure as the mud which, as already described, presents itself to the top end of that seal structure via the passage 102. As a result there is no substantial tendency for the top pressure mud presenting at the top end of the seal structure 105 to enter that seal structure and find its way into the bearing assembly. An air vent plug 127 is provided in the connector member 104 adjacent the top end of the seal structure 105 to enable the motor to be wholly filled with grease.

The purpose of the delivery of top pressure mud to the lower end of the motor via the tube 107 is to substantially prevent leakage of grease to the exterior through the bottom seal structure 112. This mud is delivered via an end fitting 128 of the tube 107 through a passage 129 in the end fitting 8 of the rotor shaft to a passage in the lowermost sleeve 117 and thence to an annular groove 131 defined in that sleeve within the seal structure 112. Thus top pressure mud is delivered to a point inside the seal structure 112 between the ends thereof and, from this point, is allowed to leak to the exterior of the motor along the inside of the seal structure to an outlet 132, such leakage being brought about by the fact that the pressure of the mud flowing up the outside of the motor will be in the order of 1,000 p.s.i. less than top pressure. As the mud delivered to the inside of the seal structure 112 is at the same top pressure as the grease delivered to the top end of the seal structure it will substantially prevent the leakage of grease through the seal structure, substantially all of the leakage which occurs being mud which is of course readily expendable.

A plug 133 is provided for feeding grease into the passages 124 for the purpose of refilling the motor with grease when it is withdrawn from a borehole to change the drill bit. The end fittings 123 and'128 of the tubes 106 and 107 are held in position by a locking sleeve 134 screw-threadedly mounted in the rotor shaft end fitting 8, and sealing lugs 135 are provided on both of the end fittings 123 and 128 which distort when the sleeve 134 is screwed tight so as to seal the end fittings relative to the fitting 8.

It will thus be seen that the invention, at least in its preferred embodiment described above and illustrated in the accompanying drawings, provides pressurised lubrication system for the rotor bearing assembly.

What I claim is:

l. A hydraulic motor for driving a rock drill, including a housing connectible to a drill-string, a rotor shaft rotatably mounted via bearing means in said housing and connectible to a drill bit to transmit driving torque thereto, sealing means between the rotor shaft and the housing arranged to oppose access of drilling mud to said bearing means, and means effective in operation to apply the pressure of drilling mud, at a high pressure substantially that at which it is delivered to the motor via the drill-string, to pressurize lubricant present in said bearing means characterized by means effective in operation to convey drilling mud, at or near to said high pressure, through the said rotor shaft to the bottom end region of the said bearing means for application to the outside of the said sealing means at said bottom end region.

2. A hydraulic motor as claimed in claim 1, wherein the said sealing means comprises inner and outer sealing devices and the said mud conveying means is arranged to convey the said mud to a point in said sealing means between said inner and outer sealing devices, a path being provided for the leakage of the said mud from the said sealing means out of the said housing.

3. A hydraulic motor as claimed in claim 2, wherein each of the said sealing devices comprises a plurality of interengaging sealing rings.

4. A hydraulic motor as claimed in claim 1, wherein the said mud conveying means is arranged to receive high pressure drilling mud directly from the drill-string without any substantial loss of pressure.

5. A hydraulic motor as claimed in claim 1, wherein the said mud conveying means comprises a conduit extending through the hollow rotor shaft, an end fitting on said conduit which end fitting is secured relative to the rotor shaft, and a passage leading along the rotor shaft to the said sealing means. 

1. A hydraulic motor for driving a rock drill, including a housing connectible to a drill-string, a rotor shaft rotatably mounted via bearing means in said housing and connectible to a drill bit to transmit driving torque thereto, sealing means between the rotor shaft and the housing arranged to oppose access of drilling mud to said bearing means, and means effective in operation to apply the pressure of drilling mud, at a high pressure substantially that at which it is delivered to the motor via the drill-string, to pressurize lubricant present in said bearing means characterized by means effective in operation to convey drilling mud, at or near to said high pressure, through the said rotor shaft to the bottom end region of the said bearing means for application to the outside of the said sealing means at said bottom end region.
 2. A hydraulic motor as claimed in claim 1, wherein the said sealing means comprises inner and outer sealing devices and the said mud conveying means is arranged to convey the said mud to a point in said sealing means between said inner and outer sealing devices, a path being provided for the lEakage of the said mud from the said sealing means out of the said housing.
 3. A hydraulic motor as claimed in claim 2, wherein each of the said sealing devices comprises a plurality of interengaging sealing rings.
 4. A hydraulic motor as claimed in claim 1, wherein the said mud conveying means is arranged to receive high pressure drilling mud directly from the drill-string without any substantial loss of pressure.
 5. A hydraulic motor as claimed in claim 1, wherein the said mud conveying means comprises a conduit extending through the hollow rotor shaft, an end fitting on said conduit which end fitting is secured relative to the rotor shaft, and a passage leading along the rotor shaft to the said sealing means. 